Slitless spectrographs are known, in which the traditional exit slit is replaced by an imaging sensor to simultaneously record an entire spectral range. A recent report using this type of device for field spectroscopy is Jenniskens “Observations of the Stardust Sample Return Capsule Entry with a Slitless Echelle Spectrograph”, Journal of Spacecraft and Rockets, Vol 47, No. 5, September-October 2010. Although described in the title as “Slitless”, it is clear to one skilled in the art that “slitless” refers to lack of an exit aperture. The named spectrograph contains an entrance aperture to achieve high resolutions of about 1000 to 2400, depending on wavelength. Use of the entrance aperture restricts light throughput, and as the object moves through the field, the spectrograph or entrance optics must be mechanically guided to stay trained on the object being studied. It would not be possible to simultaneously monitor the spectra of multiple objects in different locations within the field of view with this spectrograph. It would also be impossible to monitor the spectra of multiple objects that move in different directions.
Imaging spectrographs that can view multiple objects simultaneously are known, with a recent example described by Wang, et al., “Apparatus For Measuring Spectrographic Images”, U.S. Pat. No. 7,414,718. Although the Independent Claims do not require the use of an entrance aperture, the Detailed Drawings of the Inventive Designs (FIGS. 4 and 8) and the Summary Of The Invention teach that the device “comprises an imaging side telecentric lens for collecting optical energy from an object, an optical slit positioned behind the imaging side telecentric lens, . . . ”. To one skilled in the art, it is clear that if high resolution is desired, an entrance slit is necessary, and hence Wang, et al. include the entrance slit (130 in FIGS. 4 and 8) in the design drawings, the Summary Of The Invention, and as a Dependent Claim. Without an entrance slit, the design of Wang, et al. would not be able to achieve high spectral resolution.
Commercial examples of high resolution, compact, lightweight echelle spectrographs are known, an example being Hilliard, “Imaging Spectrograph For Multiorder Spectroscopy”, U.S. Pat. No. 6,628,383. This is the spectrograph used by Jenniskens referenced above, requiring an entrance aperture to achieve high resolution, and mechanical movement to remain trained on moving objects.
The echelle grating was developed in 1949 by George R. Harrison: “The Production of Diffraction Gratings: II. The Design of Echelle Gratings and Spectrographs,” J. Opt. Soc. Am. 39, 522-527 (1949). Pre-dating the development of the echelle, Carl King described a high resolution spectrometer design in which the entrance slit is replaced by a positive or negative lens of short focal length relative to the object and observing distance: Carl King, “A Lens to Supplant the Spectrograph Slit,” J. Opt. Soc. Am. 36, 164-164 (1946). King's development was hindered by movement of the plasma arc source being observed, creating wandering line positions at the photographic plates used to record the spectra, so he ultimately included an entrance aperture in his designs (“B” in FIG. 3 of the above reference). He also shows cylindrical condensing lenses to focus the source light onto the entrance aperture (“Cx” in FIG. 3) in the shape of a slit. Not being able to fully eliminate the entrance aperture, then, King's development has not been utilized in any known spectrometer or spectrograph design since his initial report.
A significant advance in utility, especially for field use, would be realized if the entrance aperture could be eliminated while maintaining high resolution. Simple transmission gratings are commonly used to obtain spectra without an entrance aperture, coupled directly to a camera at the field lens. These, however, are of poor resolution in the range of 50-100, typically producing 5-10 nm line widths at 500 nm. Longer paths and special scanning procedures can improve their resolution, but then compromising the ability to obtain spectra of a wide field of multiple objects. It is understood in the industry that the wider the field and more objects viewed, the poorer the available resolution. The ability to obtain high resolution spectra across a visual field of multiple objects simultaneously would have numerous advantages for identification of unknown distant luminous objects, such as multiple stars or planets in a field of view, or various terrestrial objects under observation from a distance greater than that of a laboratory bench. This would include objects that are in motion across the field of view, even in differing directions. No high resolution spectrograph design to date has been able to accommodate this typical situation encountered in field use. High resolution for this work is defined as 500 or greater. This corresponds to line widths of 1 nm or less at 500 nm. Wide field for this work is defined as greater than 1-degree, preferably greater than 10-degrees, most preferably greater than 30 degrees, to enable the spectral recording of multiple objects that might be moving, simultaneously, without the need to mechanically track the spectrograph to the object(s) motion.
What is therefore needed is a re-design of the entrance system of high resolution spectrographs in which light from a wide field is not required to pass through a small aperture. For purposes of this invention, high resolution refers to resolutions greater than 500 (1 nm FWHM at 500 nm), and small aperture refers to a restriction of less than 1 mm in smallest dimension. Preferred high resolution is greater than 1000. Preferred apertures are greater than 3 mm in smallest dimension to enable a large increase in light throughput for distant objects.